The United States and Japan will step up their defence cooperation to deal with the threat from nuclear-armed North Korea as tensions in East Asia remain high, officials from the two allies said on Thursday.

Six subs still big investment: Greens

Greens leader Richard Di Natale reckons Australia should be building six new subs, not 12 as the government and opposition prefer.

Building six subs in South Australia was still a significant investment, but the state could create a more long-term and sustainable industry base through renewable energy.

There should be a much more mature conversation about defence needs, not during an election campaign when the major parties were concerned about losing seats in SA to the Nick Xenophon Team, Senator Di Natale told the National Press Club in Canberra on Thursday.